Bone screw

ABSTRACT

In order to improve a bone screw with a shaft defining a longitudinal axis and with a head which can be brought into engagement with a bone screw receiving means of a bone plate for the releasable connection of the bone screw to the bone plate, wherein a securing element for securing a connection between the bone screw and the bone plate is provided, wherein the bone screw can be brought from a position of engagement, in which the bone screw is held on the bone plate, into a release position, in which the bone screw can be released from the bone plate, wherein the securing element can be brought from a non-securing position, in which the bone screw can be brought into the release position, into a securing position for securing the connection between the bone screw and the bone plate, in which the bone screw takes up the position of engagement, such that a bone plate can be fixed to bone parts more easily and more securely it is suggested that the securing element be supported on the bone screw so as to be movable.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/732,752 filed on Apr. 4, 2007, which is a continuation ofInternational Patent Application No. PCT/EP2005/010759 filed on Oct. 6,2005 and designating the United States, which claims priority to GermanApplication No. 10 2004 050 040, filed on Oct. 8, 2004. The contents ofall of the foregoing related applications are incorporated by referenceherein in their entireties for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates to a bone screw with a shaft defining alongitudinal axis and with a head which can be brought into engagementwith a bone screw receiving means of a bone plate for the releasableconnection of the bone screw to the bone plate, wherein a securingelement is provided for securing a connection between the bone screw andthe bone plate, wherein the bone screw can be brought from a position ofengagement, in which the bone screw is held on the bone plate, into arelease position, in which the bone screw can be released from the boneplate, wherein the securing element can be brought from a non-securingposition, in which the bone screw can be brought into the releaseposition, into a securing position for securing the connection betweenthe bone screw and the bone plate, in which the bone screw takes up theposition of engagement.

Bone screws of the type described at the outset are used in surgery tofix bone plates to bone parts of a human or animal body. The advantageof these screws is the fact that the position of engagement can belocked and so the bone plate cannot become disconnected from the bonescrew or screws when stressed. Locking elements are known, for example,which are arranged on the bone plate and are pushed over the head of thebone screw already screwed in. This has the disadvantage that a securingelement must be provided at every opening in the bone plate. If,however, not all the bone screw receiving means of the bone plate arerequired, the securing elements arranged at the bone screw receivingmeans which are not used are also, strictly speaking, superfluous. Inaddition, it must be ensured prior to the insertion of the bone platethat all the securing elements are in the non-securing position. If thisis not the case, all the securing elements must be transferred from thesecuring position into the non-securing position during the operationwhich increases the operation time unnecessarily.

SUMMARY OF THE INVENTION

In accordance with the invention, a bone screw of the type described atthe outset is suggested, wherein the securing element is supported onthe bone screw so as to be movable.

As a result of the movable supporting of the securing element on thebone screw, the number of securing elements required is merely the sameas the number of bone screws actually used for securing the bone plate.Moreover, it is ensured that each bone screw comprises its own securingelement. It is favorable, in particular, when the securing element issupported on the bone screw so as to be movable in a captive manner. Asa result, it is possible to avoid the securing element being transferredeasily and simply from the securing position into the non-securingposition and vice versa, in particular, in the case of very small bonescrews such as those used, for example, in the cervical area of thespinal column. Moreover, it cannot be lost in an operation area.

A particularly simple construction of the bone screw results when it isdesigned so as to be rotationally symmetric or essentially rotationallysymmetric in relation to the longitudinal axis.

In the position of engagement, the head preferably has a maximumexternal dimension transversely to the longitudinal axis and, in therelease position, an external dimension which is reduced in size incomparison with the maximum external dimension. This allows the head tobe brought into engagement with part of the bone plate, for example, thebone screw receiving means in the position of engagement and out ofengagement with it in the release position. The head can have anyoptional shape, for example, with a square or round cross sectiontransversely to the longitudinal axis.

Particularly with a round cross section of the head, it is advantageouswhen the head has a maximum external diameter transversely to thelongitudinal axis in the position of engagement and an external diameterwhich is reduced in size in comparison with the maximum externaldiameter in the release position. For example, the head can be pressedtogether proceeding from the position of engagement in order to transferthe bone screw into the release position or, however, also be spreadapart from the release position in order to transfer the bone screw intothe position of engagement. It would, therefore, be conceivable todesign the bone screw such that it takes up either the position ofengagement or the release position without any influence of externalforces.

A connection between the bone screw and the bone plate can be providedin a particularly simple manner when the head bears a first snap-inelement which can be interlocked with a second snap-in element arrangedon the bone plate in the position of engagement. The two snap-inelements can therefore be in engagement in the position of engagement,in particular, engage in one another in a form-locking manner.

The construction of the bone screw may be configured in a particularlysimple manner when the first snap-in element is an annular groove whichextends in circumferential direction and is open radially or an annularprojection which extends in circumferential direction and protrudesradially outwards. The annular groove may, for example, accommodate acorresponding projection or corresponding projections on the bone plate.The protruding annular projection can be brought into engagement with acorresponding groove on the bone plate in order to provide a connectionbetween the bone screw and the bone plate.

In accordance with a preferred embodiment of the invention, it may beprovided for the securing element to be held in the securing position ina basic position of the bone screw, in which no external forces act onthe bone screw. This has the advantage that a physician does not have tothink about locking the bone screw since it takes up the securingposition in a basic position. As a result, the operating time isshortened and mistakes avoided. Furthermore, the number of instrumentsrequired is also reduced since no special instrument for the securingelement is required. In order to keep the securing element in thesecuring position, the bone screw can also be designed such that noforces act on the securing element in the securing position; thesecuring element, is, therefore, held free from forces.

It is advantageous when the securing element is held in the securingposition under tension. In order to transfer the securing element fromthe securing position into the non-securing position contrary to thetensioning, a certain releasing force must, therefore, be applied. Sincethe securing element is held under tension, it is automaticallytransferred back again from the non-securing position into the securingposition. A physician need not, therefore, lock each bone screwindividually; the bone screw locks itself as a result of the specialconfiguration.

It is advantageous when at least one holding element is provided forholding the securing element in the securing position when it issupported, on the one hand, on the shaft and, on the other hand, on thesecuring element. The securing element can be arranged, in particular,in a cavity or a recess of the shaft, in which it is, in addition,protected.

Defined holding forces may be generated in different positions of thesecuring element when the holding element is an elastic element.

The construction of the bone screw is particularly simple when theholding element is a helical spring or a plate spring or stack of platesprings. A desired holding force, with which the securing element isheld in the securing position or is also held in this position undertension, may be adjusted in a simple manner.

In accordance with a preferred embodiment of the invention, it may, inaddition, be provided for the holding element to be of such a designthat an external dimension and/or a volume of the holding element isvariable due to variation of at least one state parameter in thesurroundings of the bone screw. For example, pressure can be increasedin the surroundings of the bone screw, as a result of which the volumeof the holding element is increased or decreased. As a result of thechange in the external dimension and/or the volume of the holdingelement, the securing element may be transferred from the non-securingposition into the securing position and/or vice versa.

It is favorable when the at least one state parameter is ambienttemperature, ambient pressure, osmotic pressure or humidity of thesurroundings. This allows the state of the holding element to bechanged, in particular, an outer dimension and/or a volume thereof as aresult of alteration to the specified or also additional physicalparameters in the surroundings of the bone screws.

The holding element is favorably produced from a memory metal. Inparticular, it is thus possible, for example, to transfer the securingelement from the securing position into the non-securing position as aresult of cooling or heating and vice versa, again due to heating orcooling.

In addition, the securing element can be moved in a simple manner whenthe holding element is inflatable by means of a fluid. For example, thefluid can be air, compressed air or a biocompatible liquid.

In order to be able to utilize alterations in an osmotic pressure, inparticular, in order to vary the holding element in its size, it isfavorable when the holding element has an interior space which is influid communication with the surroundings of the bone screw via asemi-permeable membrane. This allows, for example, water to be diffusedinto the interior space of the holding element as a function of asolvent concentration in the surroundings of the bone screw.

Securing of the bone screw in the position of engagement may be broughtabout particularly easily when the securing element is supported on theshaft so as to be displaceable parallel to the longitudinal axis.

In order to also facilitate, in particular, a captive supporting of thesecuring element on the bone screw, it is advantageous when the shafthas a securing element receiving means and when the securing element issupported in the securing element receiving means.

The bone screw is particularly simple to produce when the securingelement receiving means is a blind hole. The securing element can thenbe introduced, first of all, into the securing element receiving meansthrough an opening in the blind hole but cannot exit again out of thebone screw on the other side of the recess. The blind hole is preferablyarranged such that an opening thereof is arranged in the area of thehead of the bone screw.

The securing element is advantageously held in the securing elementreceiving means in a non-rotational form-locking manner. This allows aforce to be passed to the bone screw for turning the same into a bonepart via the securing element and to be transferred to the shaft of thebone screw as a result of the non-rotational form-locking design of thesecuring element receiving means. It will, in particular, be possible asa result to provide a tool adapter for a screw-in tool of the bone screwon the securing element itself.

The stability of the bone screw is increased as a whole when thesecuring element has a securing element section which is designed in theshape of an external polyhedron and when the securing element receivingmeans has a securing element receiving means section which is designedin the shape of an internal polyhedron corresponding to the externalpolyhedron. For example, the external polyhedron can be designed as asquare, hexagon or octagon. A transfer of force following a rotation ofthe securing element on the shaft of the bone screw can be optimized inthis way.

So that the securing element can be held on the bone screw in a captivemanner, it is advantageous when the securing element, after theinsertion of the securing element into the securing element receivingmeans, can be moved in an axial direction between a stop acting in adistal direction and a stop acting in a proximal direction. The securingelement can, therefore, be moved in an axial direction against the stop,which acts in a proximal direction and on which it abuts, for example,in the non-securing position. Furthermore, the securing element can bemoved against the stop acting in a distal direction and abut on it whenit takes up, for example, the securing position.

So that no tension results at the bone screw in the position ofengagement which can lead to damage thereto as a result of permanentstressing, it is advantageous when the bone screw takes up the positionof engagement without any action of external forces. This is favorable,as described, since, for example, in the case of bone screws with a headwhich is spread apart in the position of engagement, the head is alwayssubject to tension and elements which can be spread apart can break as aresult of permanent stressing.

In order to realize a transfer of the bone screw from the position ofengagement into the release position and vice versa in a simple manner,it is advantageous when the first snap-in element is supported on thebone screw so as to be movable in a radial direction. It can, therefore,be moved in a radial direction away from the longitudinal axis ortowards it. The snap-in elements may, in particular, be supported on thebone screw so as to be displaceable or pivotable.

A connection between the bone screw and the bone plate may be providedin a simple manner when the head comprises a plurality of lockingelements which can be moved transversely to the longitudinal axis andare in engagement with the bone plate in the position of engagement.

In principle, it would be conceivable to provide a different connectioninstead of a snap-in connection between the bone screw and the boneplate, for example, a bayonet connection. If, however, a snap-inconnection is desired, it is favorable when the locking elements bearthe first snap-in element. The first snap-in element can then be broughtinto engagement with the second snap-in element on the bone plate as aresult of movement of the locking element or the locking elements,respectively.

If the bone screw has a securing element receiving means, at least partof the bone screw is of a sleeve-like design. It is advantageous whenthe head has tab-like wall sections separated by slots extendingparallel to the longitudinal axis and when the wall sections form thelocking elements. In order to produce the locking elements, only thehead of the bone screw must, for example, be provided with slots. Four,five or six locking elements are advantageously provided and these canbe formed by four, five or six slots which are distributed symmetricallyover a circumference of the bone screw.

It is favorable when the locking elements take up the position ofengagement in a basic position, in which no external forces act on thebone screw. The locking elements must, therefore, be moved only for thepurpose of transferring the bone screw from the position of engagementinto the release position and are subject to tension only for this shorttime. In the position of engagement, they are, however, essentiallywithout tension and so there is no risk of the locking elements beingable to break as a result of permanent stressing.

In order to transfer the bone screw from the position of engagement intothe release position, it may be provided for the locking elements to bemoved radially in a direction towards the longitudinal axis. Therefore,the locking elements can favorably be moved radially outwards from theposition of engagement in order to insert the securing element into thesecuring element receiving means. This means, in other words, that thesecuring element spreads the locking elements apart and, with them, forexample, a head of the bone screw during its insertion into the securingelement receiving means. After insertion of the securing element intothe securing element receiving means, the locking elements can return totheir original shape, in which they take up the position of engagement.The securing element is then, with this embodiment, held in the securingelement receiving means in a captive manner. In order to remove thesecuring element from the securing element receiving means, the lockingelements must be pivoted again radially outwards in order to release theopening of the securing element receiving means to such an extent thatthe securing element can be withdrawn from it.

In order to hold the securing element on the bone screw in a captivemanner, it is favorable when a second snap-in device comprising a thirdsnap-in element and a fourth snap-in element interacting with the thirdsnap-in element is provided, when the securing element and the head eachbear a snap-in element and when the second snap-in device takes up asnap-in position after the insertion of the securing element into thesecuring element receiving means. A snap-in connection preferablyresults, therefore, when the securing element is inserted into thesecuring element receiving means. In order to release the snap-inconnection, at least one of the two snap-in elements must be movedrelative to the other. If this does not occur, the securing element isheld in the securing element receiving means in a captive manner.

A particularly simple construction of the bone screw results when thethird snap-in element is a snap-in nose which acts in a proximaldirection and projects radially outwards and when the fourth snap-inelement is a snap-in edge which projects radially inwards and acts in adistal direction. The snap-in nose and the snap-in edge can engagebehind one another in the snap-in position in order to form a snap-inconnection and thus secure the securing element in the securing elementreceiving means.

A relative movement of the third snap-in element and the fourth snap-inelement may be realized in a simple manner in that the locking elementsbear the fourth snap-in element. As a result, the fourth snap-in elementcan be moved relative to the third during movement of the lockingelements and, in addition, be brought into engagement or out ofengagement.

Advantageously, the third or the fourth snap-in element forms theproximal stop and these elements abut on one another in the securingposition. It is conceivable, in particular, for the holding element topress the securing element in a proximal direction so that the third orfourth snap-in element abuts on the proximal stop which acts in a distaldirection.

In order to facilitate the assembly of the bone screw and, inparticular, to make instruments for the assembly of the bone screwsuperfluous, it is advantageous when a slide-on surface adjoins thethird or the fourth snap-in element, the respectively other snap-inelement being able to slide on this surface during the insertion of thesecuring element into the securing element receiving means. For example,the slide-on surface can extend in a distal direction starting from thethird snap-in element arranged on the securing element so that thefourth snap-in element arranged on the head of the bone screw can slideonto it during the insertion of the securing element into the securingelement receiving means and, as a result, be moved outwards in a radialdirection until it engages behind the third snap-in element, whereby itcan be moved again in a radial direction towards the longitudinal axis.The securing element can, therefore, be secured in the securing elementreceiving means in this way.

The stability of the bone screw may be increased when the securingelement is designed in the shape of a set bolt with a bolt shaft and abolt head.

The construction of the securing element is particularly simple when thebolt shaft forms the securing element section which is designed in theshape of an external polyhedron.

In order to hold the securing element in the securing element receivingmeans so as to be captive but movable in a simple manner, it isadvantageous when the bolt head bears the third snap-in element. Forexample, the third snap-in element can be designed in the shape of anannular projection which protrudes radially outwards and forms a stopsurface which extends in a proximal direction transversely to thelongitudinal axis and is adjoined by a slide-on surface taperingconically in a distal direction.

In order to be able to screw the bone screw into a bone part in a simplemanner, it is favorable when the bone screw has a tool element adapterwhich can be brought into engagement with a screw-in tool for thescrewing in of the bone screw.

The securing element preferably bears the tool adapter. This has theadvantage that two functions can be exercised at the same time with thescrew-in tool. On the one hand, the screw-in tool can, in certainembodiments, transfer the securing element from the securing positioninto the non-securing position, for example, also contrary to thetension generated by a holding element. On the other hand, the entirebone screw can be turned in with the screw-in tool, for example, whenthe securing element is held in the securing element receiving means ina non-rotational, form-locking manner.

In principle, it would be conceivable for the tool adapter to bedesigned in the shape of a projection. It is, however, favorable whenthe tool adapter is a recess. In this respect, it may be advantageouswhen the recess is designed in the shape of a slot, a polyhedron or astar-shaped cavity.

In order to increase the stability of the bone screw, it is advantageouswhen the head and the shaft are designed in one piece.

In accordance with a preferred embodiment of the invention, it may beprovided for the shaft to be provided with an external thread. Theexternal thread can, in particular, be a self-cutting bone thread. Asecure hold of the bone screw in the bone part is ensured as a result ofthe external thread.

The object specified at the outset is also accomplished in accordancewith the invention, in an implant system comprising at least one boneplate with at least one bone screw receiving means and at least one bonescrew which can be brought into engagement with the bone screw receivingmeans in a position of engagement for the releasable connection of thebone screw to the bone plate, in that the bone screw is one of the bonescrews described above. This results in the advantages already explainedfor the connection of bone parts to the bone plate of the implantsystem, wherein the bone plate can be secured to the bone part with theat least one bone screw.

The following description of preferred embodiments of the presentinvention serves to explain the invention in greater detail inconjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of an implant system comprising a boneplate and two bone screws;

FIG. 2 shows a perspective view of a proximal end of a bone screw fromFIG. 1 prior to the insertion of a securing element;

FIG. 3 shows a partially cutaway side view of a proximal end of a bonescrew from FIG. 1 in the position of engagement;

FIG. 4 shows a partially cutaway side view of the bone screw with thesecuring element in the position of engagement;

FIG. 5 shows a view similar to FIG. 4, wherein the bone screw takes upthe release position;

FIG. 6 shows a partially cutaway side view through a second embodimentof a bone screw in the release position; and

FIG. 7 shows a view similar to FIG. 6 of a third embodiment of a bonescrew.

DETAILED DESCRIPTION OF THE INVENTION

An implant system provided altogether with the reference numeral 10 andcomprising a bone plate 12 and two self-locking bone screws 14 isillustrated in FIG. 1.

The bone plate 12 illustrated in FIG. 1 serves in the first place asillustration; it does not represent any limitation of the presentinvention. In principle, all types of bone plate, which have at leastone opening for a bone screw, would be conceivable in conjunction withthe present invention.

The bone plate 12 is flat and shaped like a clover leaf with outer edgesrounded on all sides. A groove-like recess 16, which separates,so-to-speak, two respective pairs of leaves of the bone plate 12 fromone another, is provided approximately in the center. Altogether, thebone plate 12 is designed in one piece. An opening 18 in the form of anelongated hole is provided in the area of each corner of the bone plate12, wherein longitudinal directions of the openings 18 are alignedessentially parallel to one another. A maximum extension of the openings18 is oriented transversely to the longitudinal direction of the recess16 so that a minimal inner dimension of the openings 18 essentiallyparallel to the longitudinal direction of the recess 16 ispredetermined. Inner edges 20 of the openings 18 are curved concavely inthe direction towards a center of the respective opening 18 so thatedges limiting the opening 18 project radially inwards not only on theupper side of the bone plate 12 but also on its underside. The openings18 therefore form, with their concave edges 20, recessed areas in thebone plate 20 which are essentially shaped like hollow caps.

The bone screws 14, which, in the form of a first embodiment, will beexplained in greater detail in the following in conjunction with FIGS. 1to 5, serve to secure the bone plate 12 on bone parts which are notillustrated.

The bone screw 14 is essentially designed in three parts. It comprises ashaft 22, which defines a longitudinal axis 23, a screw tip 24 on thedistal side and a screw head 26 on the proximal side, as well as asecuring element in the form of a locking pin 28 and a helical spring 30serving as a holding element. The shaft 22 is provided overapproximately three quarters of its length, starting from its screw tip24, with an external thread 32 which is preferably designed to beself-cutting. An outer contour or cover end of the screw head 26 isessentially of a cap-like design and adapted to the recessed area of theopening 18 formed by the inner edge 20, whereby a polyaxial adaptationof the screw head 26 in the recessed opening 18 is made possible.

Proceeding from its proximal end, the shaft 22 is provided with a blindhole 34 which forms a securing element receiving means and the base 36of which on the distal side points in the direction towards the proximalend of the bone screw 14. Proceeding from the base 36, a hollowcylindrical section 38 extends over approximately one third of theoverall length of the blind hole 34 and the helical spring 30, which issupported on the base 36 with its end on the distal side, is insertedinto this section. An inner hexagonal section 40, which formsapproximately a central third of the blind hole 34, adjoins the hollowcylindrical section 38. The interior diameter of the hollow cylindricalsection 38 is somewhat smaller than an interior dimension of the innerhexagonal section 40 and so a distal stop 42 formed by an annularsurface pointing in a proximal direction is formed in the area oftransition between the hollow cylindrical section 38 and the innerhexagonal section 40.

Approximately the proximal third of the shaft 22 forms the screw head 26which has an external diameter increased in size in comparison with theshaft section provided with the external thread 32. The screw head 26 isprovided on an outer side with a circumferential annular groove 44. Inorder to be able to vary the screw head 26 in its external diameter, itis provided symmetrically around its circumference with five slots 46extending parallel to the longitudinal axis 23 and so altogether fivesegments 48 of the screw head 26, which form locking elements, areformed. The segments 48 form tab-like sleeve sections which can bepivoted in a radial direction, i.e., in a direction towards thelongitudinal axis 23 or away from it. A desired pivotability may beadjusted via the depth of the annular groove 44 which forms a weakenedarea and facilitates movability of the segments 48. An opening 50 in theblind hole 34 is provided with an annular flange 52 which projectsradially inwards towards the longitudinal axis 23 and is interrupted bythe slots 46 in the same way as the annular groove 44. The annularflange 52 forms a narrowing of the internal diameter of the blind hole34. Furthermore, it has an annular stop surface 54 which points in adistal direction and forms a stop acting in a distal direction on theproximal side.

Proceeding from the inner hexagonal section 40, an interior diameter ofthe blind hole 34 widens as far as a maximum internal diameter whichextends in the interior in the area of the annular groove 44 as far asthe stop surface 54.

The locking pin 28 is, altogether, shaped in the form of a set bolt witha head 56 and a bolt shaft 58 designed in the shape of an externalhexagon and corresponding to the shape of the interior hexagonal section40. The head 56 which is of a larger size in its external diameter inrelation to the bolt shaft 58 has a snap-in lip 60 which extends incircumferential direction, projects radially outwards and comprises anannular stop surface 62 which points in a proximal direction and aslide-on surface 64 which extends in a distal direction from the outeredge of the stop surface 62 and tapers conically.

A star-shaped recess is provided on the head and this points in aproximal direction, is like a blind hole and serves as a tool receivingmeans 66 for accommodating a corresponding tool tip of a screw-in tool70, the distal end of which is illustrated in FIGS. 4 and 5.

The procedure for assembling the three-part bone screw 14 is as follows.First of all, the helical spring 30 is inserted into the blind hole 34through the opening 50 until a distal end of the helical spring 30 abutson the base 36. The next step is to push the locking pin 28 with itsbolt shaft through the opening 50 until the annular flange 52 abuts onthe slide-on surface 64. For the complete insertion of the locking pin28 into the blind hole 34, the segments 48 must now be spread radiallyoutwards. This occurs as a result of the exertion of force on thelocking pin 28 in a distal direction. The annular flange 52 on thesegments 48 slides on the slide-on surface 64 such that the segments 48are pivoted radially outwards and spring back radially inwards behindthe snap-in lip 60 as soon as the stop surface 62 engages behind thestop surface 54 on the annular flange 52. A proximal end of the helicalspring 30 abuts on the distal end of the bolt shaft 58 and presses thelocking pin 48 in a proximal direction and thus keeps the stop surfaces54 and 62 in abutment on one another. The bone screw 14 then takes upits position of engagement, as illustrated in FIGS. 1, 3 and 4.

In order to screw the bone screw 14 into a bone part which is notillustrated and to secure the bone plate 12 on the same, the bone screw14 with its shaft 22 provided with an external thread 32 is first of allpushed through the opening 18. If the tool tip 68 engages in the toolreceiving means 66, the locking pin 28 is moved in a distal directioncontrary to the spring force of the helical spring 30 until the distalend of the bolt shaft 58 abuts on the stop 42. The locking pin 28 thentakes up the non-securing position illustrated in FIG. 5.

As a result of the non-rotational, form-locking design of the innerhexagonal section 40 and the bolt shaft 58, torque introduced into thehead 56 by the tool tip 68 can be transferred to the shaft 22 of thebone screw 14. The bone screw 14 can be screwed ever deeper into thebone part, namely to such an extent until the screw head 26 with thepivotable segments 48 abuts on the upper, annular edge of the opening18. The segments 48 slide on the upper edge when the bone screw 14 isscrewed further in and are pivoted radially inwards until a proximal endof the bone screw 14, i.e., the screw head 26 can dip completely intothe recessed opening 18. The segments 48 then spring back radiallyoutwards and again take up their position of engagement. The screw head26 is held in the recessed opening 18 in this way but not yet secured.

It is possible for the screw head 26 to dip into the recessed opening 18only when the locking pin 28 takes up the non-securing positionillustrated in FIG. 5. If the bone screw 14 is seated as desired in thebone, a surgeon will remove the screw-in tool 70. As a result of therestoring force exerted by the helical spring 30 on the locking pin 28,this is moved in a proximal direction until the stop surface 62 of thesnap-in lip 60 again abuts on the stop surface 54. The head 56 of thelocking pin 28 is then seated flush between the segments 48 and forms aflat termination of the bone screw 14 on the proximal side.

The bone screw 14 takes up the position of engagement without anyinfluence from external forces and is secured in the position ofengagement by the locking pin 28 held in the securing position by meansof the helical spring 30.

In FIG. 6, a second embodiment of a bone screw provided altogether withthe reference numeral 14′ is illustrated. The bone screw 14′ differsfrom the bone screw 14 illustrated in FIGS. 1 to 5 in that instead ofthe helical spring 30 a stack 74′ of plate springs comprising altogetherelf plate springs 72′ is provided which is supported on the base 36′ onthe distal side and on the distal end of the bolt shaft 58′ on theproximal side. All the remaining parts and elements of the bone screw14′ correspond to those of the bone screw 14 and so the same parts arealso provided with the same reference numerals but with a superior caseprime (′) added in FIG. 6. The mode of operation of the bone screw 14′corresponds completely to that of the bone screw 14 described inconjunction with FIGS. 1 to 5.

A third embodiment of a bone screw provided altogether with thereference numeral 14″ is illustrated in FIG. 7. Instead of the helicalspring 30 of the bone screw 14, a holding member 76″ is provided for thebone screw 14″. All the other parts and elements of the bone screw 14″correspond to those of the bone screw 14 and so identical referencenumerals are used but with a double prime (″) added.

The holding member 76″ is variable in its volume in the embodimentillustrated. For this purpose, it has a sleeve which surrounds anelement which is variable in expansion or shape and is produced from amemory metal. As a result of changes in temperature at the bone screw,the element expands or contracts so that the locking pin 28″ can betransferred from the non-securing position illustrated in FIG. 7 intothe securing position, in which the stop surfaces 54″ and 62″ abut onone another.

1. A bone screw with a shaft defining a longitudinal axis and a headadapted to be brought into engagement with a bone screw receiving meansof a bone plate for the releasable connection of the bone screw to thebone plate, wherein a securing element is provided for securing aconnection between the bone screw and the bone plate, wherein the bonescrew is adapted to be brought from a position of engagement, the bonescrew being held on the bone plate in said position, into a releaseposition, the bone screw being releasable from the bone plate in saidrelease position, wherein the securing element is adapted to be broughtfrom a non-securing position, the bone screw being adapted to be broughtinto the release position in said non-securing position, into a securingposition for securing the connection between the bone screw and the boneplate, the bone screw taking up the position of engagement in saidsecuring position, wherein the securing element is supported on the bonescrew so as to be movable, the bone screw further comprising at leastone holding element, the at least one holding element exerting a biasingforce on the securing element in the direction of the longitudinal axisto hold the securing element in the securing position, the head bearinga first snap-in element adapted to be interlocked with a second snap-inelement arranged on the bone plate in the position of engagement, theshaft having a hole, the securing element being supported in the hole,the head comprising a plurality of locking elements movable transverselyto the longitudinal axis, said locking elements being in engagement withthe bone plate in the position of engagement, the bone screw furthercomprising a second snap-in device comprising a third snap-in elementand a fourth snap-in element interacting with the third snap-in element,wherein the securing element and the head each bear a snap-in elementand wherein the second snap-in device takes up a snap-in position afterthe insertion of the securing element into the hole, wherein the lockingelements bear the fourth snap-in element.
 2. A bone screw as defined inclaim 1, wherein the bone screw is generally rotationally symmetric inrelation to the longitudinal axis.
 3. A bone screw as defined in claim1, wherein in the position of engagement the head has a maximum externaldimension transversely to the longitudinal axis and in the releaseposition an external dimension reduced in size in comparison with themaximum external dimension.
 4. A bone screw as defined in claim 3,wherein in the position of engagement the head has a maximum externaldiameter transversely to the longitudinal axis, and in the releaseposition an external diameter reduced in size in comparison to themaximum external diameter.
 5. A bone screw as defined in claim 1,wherein the first snap-in element is an annular groove extending incircumferential direction and being open radially or an annularprojection protruding radially outward and extending in circumferentialdirection.
 6. A bone screw as defined in claim 1, wherein the securingelement is held in the securing position in a basic position of the bonescrew, with no external forces acting on the bone screw in said basicposition.
 7. A bone screw as defined in claim 6, wherein the securingelement is held in the securing position by an abutment.
 8. A bone screwas defined in claim 6, wherein said holding element is supported on atleast one of the shaft and on the securing element.
 9. A bone screw asdefined in claim 8, wherein the holding element is an elastic element.10. A bone screw as defined in claim 8, wherein the holding element isdesigned in such a manner that an external dimension and/or a volume ofthe holding element is variable due to alteration to at least one stateparameter in the surroundings of the bone screw.
 11. A bone screw asdefined in claim 10, wherein the at least one state parameter is ambienttemperature, ambient pressure, osmotic pressure or humidity of thesurroundings.
 12. A bone screw as defined in claim 11, wherein theholding element is produced from a memory metal.
 13. A bone screw asdefined in claim 11, wherein the holding element has an interior spacein fluid communication with the surroundings of the bone screw via asemi-permeable membrane.
 14. A bone screw as defined in claim 1, whereinthe securing element is supported on the shaft so as to be displaceableparallel to the longitudinal axis.
 15. A bone screw as defined in claim1, wherein the hole is a blind hole.
 16. A bone screw as defined inclaim 1, wherein the securing element is held in the hole in anon-rotational form-locking manner.
 17. A bone screw as defined in claim1, wherein the securing element has a securing element section designedin the shape of an external polyhedron and wherein the hole has a holesection designed in the shape of an internal polyhedron corresponding tothe external polyhedron.
 18. A bone screw as defined in claim 1, whereinfollowing the insertion of the securing element into the hole thesecuring element is movable in an axial direction between a stop actingin a distal direction and a stop acting in a proximal direction.
 19. Abone screw as defined in claim 1, wherein the bone screw takes up theposition of engagement without any action of external forces.
 20. A bonescrew as defined in claim 1, wherein the first snap-in element issupported on the bone screw so as to be movable in a radial direction.21. A bone screw as defined in claim 1, wherein the locking elementsbear the first snap-in element.
 22. A bone screw as defined in claim 1,wherein the head has tab-like wall sections separated by slots extendingparallel to the longitudinal axis and wherein the wall sections form thelocking elements.
 23. A bone screw as defined in claim 1, wherein thelocking elements take up the position of engagement in a basic position,no external forces acting on the bone screw in said basic position. 24.A bone screw as defined in claim 1, wherein for the insertion of thesecuring element into the hole the locking elements are movable radiallyoutwards from the position of engagement.
 25. A bone screw as defined inclaim 1, wherein the third snap-in element is a snap-in nose acting in aproximal direction and projecting radially outwards and wherein thefourth snap-in element is a snap-in edge projecting radially inwards andacting in a distal direction.
 26. A bone screw as defined in claim 1,wherein the third or the fourth snap-in element forms the proximal stopand said elements abut on one another in the securing position.
 27. Abone screw as defined in claim 1, wherein a slide-on surface adjoins thethird or the fourth snap-in element, the respectively other snap-inelement being able to slide on said surface during the insertion of thesecuring element into the hole.
 28. A bone screw as defined in claim 1,wherein the securing element is designed in the shape of a set bolt witha bolt shaft and a bolt head.
 29. A bone screw as defined in claim 28,wherein the bolt shaft forms the securing element section designed inthe shape of an external polyhedron.
 30. A bone screw as defined inclaim 28, wherein the bolt head bears the third snap-in element.
 31. Abone screw as defined in claim 1, wherein the bone screw has a toolelement adapter adapted to be brought into engagement with a screw-intool for the screwing in of the bone screw.
 32. A bone screw as definedin claim 31, wherein the securing element bears the tool adapter.
 33. Abone screw as defined in claim 31, wherein the tool adapter is a recessin the shape of a slot, a polyhedron or a star-shaped cavity.
 34. A bonescrew as defined in claim 1, wherein the head and the shaft are designedin one piece.
 35. A bone screw as defined in claim 1, wherein the shaftis provided with an external thread with a self-cutting external thread.36. A bone screw with a shaft defining a longitudinal axis and a headadapted to be brought into engagement with a bone screw receiving meansof a bone plate for the releasable connection of the bone screw to thebone plate, wherein a securing element is provided for securing aconnection between the bone screw and the bone plate, wherein the bonescrew is adapted to be brought from a position of engagement, the bonescrew being held on the bone plate in said position, into a releaseposition, the bone screw being releasable from the bone plate in saidrelease position, wherein the securing element is adapted to be broughtfrom a non-securing position, the bone screw being adapted to be broughtinto the release position in said non-securing position, into a securingposition for securing the connection between the bone screw and the boneplate, the bone screw taking up the position of engagement in saidsecuring position, wherein the securing element is supported on the bonescrew so as to be movable, the bone screw further comprising at leastone holding element, the at least one holding element exerting a biasingforce on the securing element in the direction of the longitudinal axisto hold the securing element in the securing position, the securingelement being held in the securing position in a basic position of thebone screw, with no external forces acting on the bone screw in saidbasic position, said holding element being supported on at least one ofthe shaft and on the securing element, the holding element beingdesigned in such a manner that an external dimension and/or a volume ofthe holding element is variable due to alteration to at least one stateparameter in the surroundings of the bone screw, the at least one stateparameter being ambient temperature, ambient pressure, osmotic pressureor humidity of the surroundings, wherein the holding element isinflatable by means of a fluid.
 37. An implant system comprising atleast one bone plate with at least one bone screw receiving means and atleast one bone screw adapted to be brought into engagement with the bonescrew receiving means in a position of engagement for the releasableconnection of the bone screw to the bone plate, wherein the bone screwhas a shaft defining a longitudinal axis and a head adapted to bebrought into engagement with a bone screw receiving means of a boneplate for the releasable connection of the bone screw to the bone plate,wherein a securing element is provided for securing a connection betweenthe bone screw and the bone plate, wherein the bone screw is adapted tobe brought from a position of engagement, the bone screw being held onthe bone plate in said position, into a release position, the bone screwbeing releasable from the bone plate in said position, wherein thesecuring element is adapted to be brought from a non-securing position,the bone screw being adapted to be brought into the release position insaid non-securing position, into a securing position for securing theconnection between the bone screw and the bone plate, the bone screwtaking up the position of engagement in said securing position, whereinthe securing element is supported on the bone screw so as to be movable,the bone screw further comprising at least one holding element, the atleast one holding element exerting a biasing force on the securingelement in the direction of the longitudinal axis to hold the securingelement in the securing position, the head bearing a first snap-inelement adapted to be interlocked with a second snap-in element arrangedon the bone plate in the position of engagement, the shaft having ahole, the securing element being supported in the hole, the headcomprising a plurality of locking elements movable transversely to thelongitudinal axis, said locking elements being in engagement with thebone plate in the position of engagement, the bone screw furthercomprising a second snap-in device comprising a third snap-in elementand a fourth snap-in element interacting with the third snap-in element,wherein the securing element and the head each bear a snap-in elementand wherein the second snap-in device takes up a snap-in position afterthe insertion of the securing element into the hole, wherein the lockingelements bear the fourth snap-in element.